"What size inverter do I need?" is asked about two different machines, and mixing them up is how people end up with the wrong purchase:

1. A standalone inverter — the box that converts a battery bank's DC to AC for an off-grid solar system, RV, boat, or cabin

2. The inverter in a portable power station — already built in, already sized; the question becomes "is the station's inverter big enough for my loads?"

This guide answers both. You'll get the three-rule sizing method, the load table with real startup-surge numbers, three worked examples from cabin to whole house, and a free embeddable calculator that does the math for you.

TL;DR — the three rules:

1. Continuous: sum the watts of everything running at the same time→ your inverter's rated output must exceed that

2. Surge: the biggest motor's startup spike (2–6× its running watts) must fit the inverter's surge rating

3. Margin: add 10–20% to the continuous total so the inverter never lives at 100%

What "Size" Actually Means: Two Numbers, Not One

Every inverter has two ratings, and buyers who watch only the first one get burned:

●Continuous (rated) watts — what it can deliver indefinitely

●Surge (peak) watts — what it can deliver for a few seconds, for motor startups

A "2,000W inverter" with a 3,000W surge rating will start a well pump; one sagging to a 2,500W surge rating may not. Sizing is a two-number problem, and the surge number is where most first-time systems fail.

And the waveform matters first of all: in 2026, pure sine wave is the only sensible default for anything with electronics. Modified sine wave inverters (the cheap ones) are still fine for a space heater or incandescent bulbs, but they can overheat and damage fans, laptops, medical devices, and variable-speed compressors. If a quote says "modified sine wave" and the loads include modern electronics, it's the wrong inverter, full stop.

The Three-Step Sizing Method

Step 1 — The continuous sum. List every load that can run simultaneously (not everything you own — what's on at the same time) and add the watts.

Step 2 — The surge check. For every motor/compressor load, compute the startup spike. Rules of thumb:

The inverter's surge rating must clear the worst simultaneous spike — usually just the single largest motor, since two compressors rarely start in the same second.

Step 3 — The margin. Multiply the continuous sum by 1.1–1.2. An inverter pinned at 100% runs hot, sheds efficiency, and ages faster; the 10–20% headroom is what keeps it at 80–90% — its comfortable, efficient zone (modern pure sine wave inverters run 90–96% efficient, peaking near 50–75% load).

The Load Table (with Surge)

Read it two ways: the running column sets your continuous rating, the surge column sets your surge rating. Notice the water heater — no motor, no surge, but 3,000W of pure continuous draw that by itself sets the floor of a whole-home inverter.

Three Worked Examples

1. The small cabin (2,000W inverter). Simultaneous worst case: fridge 200W + microwave 1,000W (briefly) + lights 50W + laptop 65W + router 15W = 1,330W continuous. Surge: the fridge's ~800 W spike (the microwave has no motor). 1,330 × 1.2 = ~1,600W → a 2,000W / 3,000W-surge inverter covers it with room. ✅

2. The off-grid home (5,000W inverter). Fridge 200W + oven element 2,000W + water heater 3,000W (not simultaneous with the oven — pick the heavier of the two as the design case) + lights/router/fan 150W. Design case: water heater 3,000 + fridge 200 + base 150 = 3,350W → ×1.2 = 4,000W → a 5,000W / 10,000W-surge inverter is the clean fit (the 10k surge clears the fridge and any tool startup). ✅

3. The whole house with AC (8–10,000W inverter). Add a 12,000 BTU AC (1,400W running, 3,000W surge) to Example 2's design case: 3,350 + 1,400 = 4,750W → ×1.2 = 5,700W → and the worst surge is now the AC's 3,000W, which fits a 10,000W-surge unit. This is where most whole-home off-grid systems land: 8,000–10,000W inverter (the [per-kWh cost guide] covers what that energy actually costs to produce).

Free Inverter Sizing Calculator (Embeddable)

Drop this self-contained calculator into a Shopify/WordPress page — enter your simultaneous loads, mark which are motors, and it returns the continuous total, the worst-case surge, and the recommended inverter tier:

<div id="inv-calc" style="max-width:560px;margin:0 auto;font-family:inherit">
  <style>#inv-calc{border:1px solid #ddd;border-radius:12px;padding:20px}
  #inv-calc input,#inv-calc select{width:100%;padding:8px;margin:4px 0 12px;box-sizing:border-box}
  #inv-calc .row{display:flex;gap:8px}#inv-calc .row>div{flex:1}
  #inv-calc .out{font-size:15px;line-height:1.6}
  #inv-calc button{width:100%;padding:10px;border:0;border-radius:8px;background:#1a7f37;color:#fff;font-size:15px;cursor:pointer}
  #inv-calc .big{font-size:20px;font-weight:700}</style>
  <h3 style="margin-top:0">Inverter Size Calculator</h3>
  <div class="row"><div><label>Load name</label><input id="name" placeholder="e.g. Fridge"></div>
  <div><label>Running watts</label><input id="w" type="number" value="200"></div>
  <div><label>Type</label><select id="t"><option value="1">No motor</option><option value="3">Motor (2–3x)</option><option value="4">Compressor (3–4x)</option><option value="6">Hard-start (4–6x)</option></select></div></div>
  <button onclick="inv.add()">Add load</button>
  <div id="list" class="out" style="margin-top:12px"></div>
  <button onclick="inv.run()" style="margin-top:8px;background:#0b5ed7">Size my inverter</button>
  <div id="res" class="out" style="margin-top:12px"></div>
</div>
<script>
const inv={items:[],tiers:[1000,2000,3000,5000,8000,10000],
 add(){const d=document.getElementById('name').value||'Load',
  w=+document.getElementById('w').value||0,m=+document.getElementById('t').value;
  if(w>0){this.items.push({d,w,m});this.render();}},
 render(){const l=document.getElementById('list');
  l.innerHTML=this.items.map((x,i)=>`<div style="display:flex;justify-content:space-between">
  <span>${x.d}: ${x.w}W continuous, ${x.w*x.m}W surge</span>
  <span style="cursor:pointer;color:#c0392b" onclick="inv.del(${i})">✕</span></div>`).join('')||'<em>No loads yet — add what runs at the SAME time</em>';},
 del(i){this.items.splice(i,1);this.render();},
 run(){if(!this.items.length){document.getElementById('res').innerHTML='<em>Add at least one load.</em>';return;}
  const cont=this.items.reduce((s,x)=>s+x.w,0);
  const worstSurge=Math.max(...this.items.map(x=>x.w*x.m));
  const need=Math.ceil(cont*1.2);
  let tier=this.tiers.find(t=>t>=need)||10000;
  const surgeOk=worstSurge<=tier*2;
  document.getElementById('res').innerHTML=
   `<div><b>Continuous total:</b> ${cont} W</div>
    <div><b>With 20% margin:</b> ${need} W</div>
    <div><b>Worst-case surge:</b> ${worstSurge} W</div>
    <div class="big">→ Recommended: ${tier.toLocaleString()}W inverter</div>
    <div style="color:${surgeOk?'#1a7f37':'#c0392b'}">${surgeOk
     ?'✓ Worst surge fits a typical 2x-surge unit at this tier.'
     :'⚠️ Worst surge exceeds 2x this tier — pick a model with a higher surge rating (check the spec, not the tier name).'}</div>
    <div style="margin-top:8px">Battery-side draw ≈ ${need}W ÷ 0.93 efficiency → size your bank/solar for ~${Math.round(need/0.93)}W of DC.</div>`;}};
</script>

If You Meant the Inverter Inside a Power Station

Same physics, different shopping: you're not buying the inverter, you're checking the station's rating. Two numbers on the spec sheet:

●Rated (continuous) output ≥ your simultaneous load total (with margin)

●Surge output ≥ your biggest motor's startup spike

That's the entire station-sizing conversation for output — our [1,000W chart] and [Wh calculator] are the companion pieces (this article sets the watts; the Wh calculator sets the hours). The surge rule bites most often at the fridge and the AC — the two loads where "the station refused to start" stories usually begin (see the [AC guide]).

Frequently Asked Questions

What size inverter do I need for a house? For essential-circuit off-grid (fridge, lights, router, small appliances): 2,000–3,000W. Add an electric oven or water heater: 5,000W. Add AC: 8,000–10,000W. Size from your simultaneous worst case × 1.2, then verify the surge rating against your largest motor.

Do microwaves and toasters need surge headroom? No — they're resistive loads with no motor, so they draw their rated watts flat from second one. Surge headroom exists for motors and compressors (fridges, AC, pumps, drills), not for heating elements.

What's the difference between a 2000W and a 2000VA inverter? Watts is the real power; VA (volt-amperes) includes reactive power, related by the power factor. At a typical 0.8–1.0 factor, 2,000VA ≈ 1,600–2,000W. Size on watts, and treat a "2000VA" claim as roughly a 1,600W-class inverter unless it states 2,000W.

Can I run an inverter at 100% of its rating all the time? You shouldn't. Sustained 100% load runs the unit hot, at the worst point of its efficiency curve, and ages it faster — the 10–20% sizing margin exists precisely so the inverter lives at 80–90%, where modern units hit 90–96% efficiency.

Pure sine wave or modified sine wave? Pure sine wave, unless the only loads are heaters and incandescent bulbs. Modified sine wave can overheat and damage modern electronics (laptops, medical devices, variable-speed compressors) — it's a 1990s compromise that doesn't survive contact with 2026 devices.

Final Thoughts

Inverter sizing is three numbers and one habit: sum the simultaneous watts, clear the biggest surge, add 20%, and always check the surge rating — not just the nameplate. Do that and the category's most common failure (the inverter that refuses one specific appliance) stops happening, because the failure is a surge problem masquerading as a size problem. Run your real load list through the calculator above, pick the tier with headroom rather than at the edge, and the inverter becomes the part of the system you never think about — which is exactly what correctly-sized equipment does.

[Optional CTA: Every NEJoye station publishes both numbers — rated and surge output — in the open spec sheet, sized to clear compressor starts → NEJoye Power Inverter]